When the pmd_devmap() checks were added by:
commit 5c7fb56e5e3f ("mm, dax: dax-pmd vs thp-pmd vs hugetlbfs-pmd")
to add better support for DAX huge pages, they were all added to the end of
if() statements after existing pmd_trans_huge() checks. So, things like:
- if (pmd_trans_huge(*pmd))
+ if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd))
When further checks were added after pmd_trans_unstable() checks by:
commit 7267ec008b5c ("mm: postpone page table allocation until we have page
to map")
they were also added at the end of the conditional:
+ if (pmd_trans_unstable(fe->pmd) || pmd_devmap(*fe->pmd))
This ordering is fine for pmd_trans_huge(), but doesn't work for
pmd_trans_unstable(). This is because DAX huge pages trip the bad_pmd()
check inside of pmd_none_or_trans_huge_or_clear_bad() (called by
pmd_trans_unstable()), which prints out a warning and returns 1. So, we do
end up doing the right thing, but only after spamming dmesg with suspicious
looking messages:
mm/pgtable-generic.c:39: bad pmd ffff8808daa49b88(84000001006000a5)
Reorder these checks in a helper so that pmd_devmap() is checked first,
avoiding the error messages, and add a comment explaining why the ordering
is important.
Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Fixes: commit 7267ec008b5c ("mm: postpone page table allocation until we have page to map")
Cc: stable@xxxxxxxxxxxxxxx
---
Changes from v1:
- Break the checks out into the new pmd_devmap_trans_unstable() helper and
add a comment about the ordering (Dave). I ended up keeping this helper
in mm/memory.c because I didn't see an obvious header where it would
live happily. pmd_devmap() is either defined in
arch/x86/include/asm/pgtable.h or in include/linux/mm.h depending on
__HAVE_ARCH_PTE_DEVMAP and CONFIG_TRANSPARENT_HUGEPAGE, and
pmd_trans_unstable() is defined in include/asm-generic/pgtable.h.
- Add a comment explaining why pte_alloc_one_map() doesn't suffer from races.
This was the result of a conversation with Dave Hansen.
---
mm/memory.c | 40 ++++++++++++++++++++++++++++++----------
1 file changed, 30 insertions(+), 10 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index 6ff5d72..2e65df1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3029,6 +3029,17 @@ static int __do_fault(struct vm_fault *vmf)
return ret;
}
+/*
+ * The ordering of these checks is important for pmds with _PAGE_DEVMAP set.
+ * If we check pmd_trans_unstable() first we will trip the bad_pmd() check
+ * inside of pmd_none_or_trans_huge_or_clear_bad(). This will end up correctly
+ * returning 1 but not before it spams dmesg with the pmd_clear_bad() output.
+ */
+static int pmd_devmap_trans_unstable(pmd_t *pmd)
+{
+ return pmd_devmap(*pmd) || pmd_trans_unstable(pmd);
+}
+
static int pte_alloc_one_map(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
@@ -3052,18 +3063,27 @@ static int pte_alloc_one_map(struct vm_fault *vmf)
map_pte:
/*
* If a huge pmd materialized under us just retry later. Use
- * pmd_trans_unstable() instead of pmd_trans_huge() to ensure the pmd
- * didn't become pmd_trans_huge under us and then back to pmd_none, as
- * a result of MADV_DONTNEED running immediately after a huge pmd fault
- * in a different thread of this mm, in turn leading to a misleading
- * pmd_trans_huge() retval. All we have to ensure is that it is a
- * regular pmd that we can walk with pte_offset_map() and we can do that
- * through an atomic read in C, which is what pmd_trans_unstable()
- * provides.
+ * pmd_trans_unstable() via pmd_devmap_trans_unstable() instead of
+ * pmd_trans_huge() to ensure the pmd didn't become pmd_trans_huge
+ * under us and then back to pmd_none, as a result of MADV_DONTNEED
+ * running immediately after a huge pmd fault in a different thread of
+ * this mm, in turn leading to a misleading pmd_trans_huge() retval.
+ * All we have to ensure is that it is a regular pmd that we can walk
+ * with pte_offset_map() and we can do that through an atomic read in
+ * C, which is what pmd_trans_unstable() provides.
*/
- if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd))
+ if (pmd_devmap_trans_unstable(vmf->pmd))
return VM_FAULT_NOPAGE;
+ /*
+ * At this point we know that our vmf->pmd points to a page of ptes
+ * and it cannot become pmd_none(), pmd_devmap() or pmd_trans_huge()
+ * for the duration of the fault. If a racing MADV_DONTNEED runs and
+ * we zap the ptes pointed to by our vmf->pmd, the vmf->ptl will still
+ * be valid and we will re-check to make sure the vmf->pte isn't
+ * pte_none() under vmf->ptl protection when we return to
+ * alloc_set_pte().
+ */
vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
&vmf->ptl);
return 0;
@@ -3690,7 +3710,7 @@ static int handle_pte_fault(struct vm_fault *vmf)
vmf->pte = NULL;
} else {
/* See comment in pte_alloc_one_map() */
- if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd))
+ if (pmd_devmap_trans_unstable(vmf->pmd))
return 0;
/*
* A regular pmd is established and it can't morph into a huge
--
2.9.4